Communications equipment that is powered by a battery, such as cellular telephone subscriber units, is required to operate for as long as possible from a charged battery. Some radio frequency (RF) power amplifiers (PA) used in battery powered communications equipment are configured to operate with a power supply that produces an output voltage that dynamically tracks the envelope of the RF input signal to the RF PA. This results in the power supply output voltage tracking the envelope of the amplified RF signal at the output of the RF PA. Depending on the state of the reference input signal, the output of the power supply can be higher or lower than the battery voltage. In addition to envelope tracking, the peak envelope voltage of the power supply can alternatively be configured to track the average power of the RF PA. As the voltage supplied by a battery in such equipment drops, such as is caused by depletion of energy stored in the battery, the current demand by the power supply while producing a voltage higher than that produced by the battery will correspondingly increase. The output voltage of the battery is further reduced due to the increased current being passed through the internal battery resistance. The net effect of these factors is that a very large current peak by the radio frequency power amplifier causes battery monitoring circuits to determine that the battery voltage has dropped below an operating threshold, thereby triggering an early radio shutdown that leaves considerable energy in the battery.
Some radio frequency amplifiers, including radio frequency amplifiers used with time division, multiple access (TDMA) communications equipment, exhibit short periods with relatively high supply current peaks. These current peaks are exacerbated by the increased current demand caused by the constant voltage power supply. This increased current being drawn through the internal battery resistance can cause a large drop in the output voltage of the battery. Many battery operated communications equipment designs incorporate a battery monitor that will shut down the equipment, or at least the transmitter, when the battery voltage drops below a minimum operating level. Such high current spikes and the attendant decrease in battery output voltage can cause the radio to reset and on-going communications can be ungracefully terminated. A further problem is that the low battery output voltage can cause the voltage regulators that supply the radio frequency circuits, such as a VCO/Synthesizer, may no longer produce a regulated output and may cause the VCO to become unlocked and cause radio frequency signal splattering that could potentially violate radio frequency transmission regulations.
Therefore a need exists to overcome the problems with the prior art as discussed above.